I feel you on the mushrooms, but I disagree on the onions. But, to each their own, continue picking off/not ordering onions on stuff as long as it makes you happy. I know there's a good amount of stuff I'm not a fan of that people would look at me cock-eyed about. For some reason when I was younger I had some strange penchant about not like cheese on anything, except pizza of course. I don't know why I only chose to eat cheese on pizza, I'm not saying this made sense, all I know is that I love it now.
I think/know that the delicious thing that changed my mind was the cheesy garlic bread at black angus...soooo fucking good.